1. Field of the Invention
Aspects of the present invention relate to a recycler for a direct methanol fuel cell (DMFC), which uses methanol as a fuel, to recycle unreacted methanol recovered from a stack and H2O, and a method of operating the same.
2. Description of the Related Art
A fuel cell is an electric generator that changes chemical energy of a fuel into electrical energy through a chemical reaction. The fuel cell continuously generates electricity as long as fuel is supplied. Of the fuel cells, a direct methanol fuel cell (DMFC) is an apparatus that uses methanol as a fuel to generate electricity through a reaction between the fuel directly fed to an anode and oxygen supplied to a cathode of the DMFC. In the anode of the DMFC, electrons are generated through Chemical Reaction 1 as indicated below, and the electrons move to the cathode along a moving path and generate H2O through Chemical Reaction 2 as indicated below. When a load is applied to the moving path, work can be done using the generated electricity.CH3OH+H2OCO2+6H++6e−  [Chemical Reaction 1]3/2O2+6H++6e−3H2O  [Chemical Reaction 2]
Methanol can be supplied to the anode by pumping liquid state methanol, and such DMFC is referred to as an active type DMFC. Or, vaporized methanol may be induced to flow into the anode as methanol vaporizes at room temperature, and such DMFC is referred to as a passive type DMFC. Here, an active type DMFC will be described.
FIG. 1 is a functional block diagram of the configuration of an active DMFC. A single assembly of an anode and a cathode, where the Chemical Reactions 1 and 2 occur, cannot generate a sufficient voltage of electricity, and thus, a stack 20, formed by stacking a plurality of the single assemblies, is used. In the stack 20, a plurality of unit cells are stacked such that in each of the unit cells, an anode and a cathode are formed respectively on opposite sides of an electrolyte membrane, and thus, a large power is output by adding the electricity generated from each of the unit cells. The active DMFC includes an air pump 60 to supply air as a source of oxygen to the cathode and a cartridge 30 where methanol to be supplied to the anode is stored. In the cartridge 30, high concentration methanol, for example, 100% methanol, is stored. Also, the active DMFC includes a storage tank 70 to store a diluted fuel, having a concentration of 0.5 to 2M, to be supplied to the anode of the stack 20 through a supply pump 50. The diluted fuel is made by adding water to the high concentration methanol supplied from the cartridge 30 through a fuel pump 40 to obtain the diluted fuel with a concentration of 0.5 to 2M.
The active DMFC includes a heat exchanger 80 to decrease the temperature of a gas-liquid mixture discharged from the stack 20. That is, the heat exchanger 80 condenses steam in the gas-liquid mixture discharged from the stack 20 by decreasing the high temperature of the gas-liquid mixture. Also, the active DMFC includes a recycler 10 to recycle unreacted methanol that is discharged from the stack 20 after having generated electricity and H2O, which is a by-product from the electricity generation reaction. The recycler 10 is also referred to as a gas-liquid separator since the recycler 10 separates unreacted methanol and water (by-product) from the gas-liquid mixture recovered from the stack 20 to reuse the unreacted methanol and water to dilute the high concentration methanol. Instead of including the recycler 10, low concentration methanol can be stored in the cartridge 30. However, in such case, the capacity of the cartridge 10 must be very large. Thus, as described above, high concentration methanol is stored in the cartridge 30 and then is gradually supplied to the storage tank 70 to be diluted.
FIGS. 2A and 2B are cross-sectional views of the structure of the conventional recyclers 10 employed in the active DMFC of FIG. 1. In the conventional recycler 10a of FIG. 2A, a gas and liquid are separated by gravity; that is, the gas is discharged through an upper port 11a of a housing 11 and the liquid, which is denser than the gas, is discharged through a lower port 11b of the housing 11. The liquid includes methanol that did not react in the stack 20, and water produced as a by-product, and the gas includes air supplied as an oxygen source and CO2 generated from a chemical reaction at the anode. The separated liquid is appropriately mixed with high concentration methanol supplied to the storage tank 70 from the cartridge 30, so as to have an appropriate low concentration methanol needed at the stack 20 for electricity generation, and as described above, the mixture is re-supplied to the stack by the supply pump 50. The structure of the recycler 10a of FIG. 2A has an advantage of structural simplicity as recycler 10a uses gravity to function; however, the lower port 11b, through which the liquid is discharged, must be disposed in the direction of gravity. Thus, recycler 10a is limited by the direction of gravity. Active DMFCs are more commonly being used in various mobile apparatuses, and as such, the lower port 11b of the conventional recycler 10a may be moved such that the lower port 11b is disposed in a direction opposite to the direction of gravity when the mobile apparatus is being used. In such case, the conventional recycler 10a cannot appropriately perform the function of gas-liquid separation.
The structure of FIG. 2B is of another conventional recycler 10b that is designed to address the above problem. That is, as depicted in FIG. 2B, the conventional recycler 10b does not use gravity to function, instead, a hydrophobic membrane 12a is installed in one port of a housing 12 and a hydrophilic membrane 12b is installed in another port of the housing 12 so that gas can be discharged through the hydrophobic membrane 12a and liquid can be discharged through the hydrophilic membrane 12b. In this way, the gas-liquid separation can be achieved regardless of the direction of gravity, and thus, the active DMFC can be applied to a mobile apparatus. However, as time passes, the characteristics of the hydrophobic and hydrophilic membranes 12a and 12b, degrade leading to a liquid leakage, and thus, the recycler 10b having the hydrophobic and hydrophilic membranes 12a and 12b are difficult to commercialize the active DMFC.
Accordingly, there is a need to develop a new recycler structure that can perform an efficient gas-liquid separation function regardless of the direction of gas-liquid separation and without using materials such as membranes of which the performances rapidly degrade over time.